The theme is "Wow cool past Rider suits!"

The theme is that of journey and identity; what truly makes you you. Themes of Tsukasa's identity are rampant even when not explicitly stated -- that shot in the opening of him falling to his side as all 9 Riders' helmets flash by on top of him isn't a mistake; it's pretty critical to what the show's about.

Decade obviously had some trouble finding a consistent voice on what exactly is was saying about these things, but I'd definitely concur that it was still always trying to keep on these topics. The photography motif (which, remember, extends all the way to the Decadriver's design) fits into this in... well, probably a few ways, honestly. I feel like there's room for interpretation, but I've always seen the key being that a photograph is essentially proof of something's existence, and of course, Tsukasa takes pictures of just about everything except himself... and also they all come out weird... and all that means... something. Maybe. Decade is a hard show to get a grasp on sometimes. :p

Looks like proof of my theory to me.

Onore dickeido.

It probably is about identity to a degree but I also doubt that much thinking went into it either.

It was the catalyst that began the phase 2 of Heisei Riders so they no longer have to compete with its super sentai siblings for toy sales. And instead have us fight against each other regarding the number theory. So bottom line the theme is Destroy to create, create to destroy.

Evolt should have had an evil catchphrase after becoming a Rider, to contrast with Sento. Something like "saa,, metsubo wo hajimeyo ka...". Come to think of it, 'Evol(ving)' is sort of an opposite to Building something, isn't it? ...I'm trying to hard to set up more parallels than necessary for what is basically a dragonball villain.

I thought he already had a catchphrase? I distinctly remember hearing a man's voice shout HELL YEAH every time he showed up onscreen. I always thought it was a little weird that the catchphrase was in English, and it definitely sounded a bit more Midwestern America than Hipster Dad's Japanese accent. Still! I certainly heard it a bunch over the run of Build!

Can't say I recall that! You might have gotten it confused with my excited wailing that was so powerful at the time of airing that it retroactively went back in time and got picked up during episode filming too.